"After nearly half a century of research in planetary and climate science for NASA, James E. Hansen is retiring on Wednesday to pursue his passion for climate activism without the hindrances that come with government employment," The New York Times' Dot Earth blog writes.

"After spending three or four years interacting with the Bush administration, I realized they were not taking any actions to deal with climate change," he told NPR in '09. "So, I decided to give one talk, and then it snowballed into another talk and eventually to even protesting and getting arrested."

He became, as The Washington Post says, "NASA's most famous climate scientist." It rounds up some of the "more notable moments of his scientific career," from the 1988 testimony that was "one of the first and clearest public statements on global warming" to his arrest in February of this year at a protest outside the White House. He was among those objecting to the proposed Keystone XL Pipeline that would carry crude oil from Canada to the U.S. Gulf Coast.

"Perhaps the biggest fight of Dr. Hansen's career broke out in late 2005, when a young political appointee in the administration of George W. Bush began exercising control over Dr. Hansen's statements and his access to journalists. Dr. Hansen took the fight public and the administration backed down."

Now 72, Hansen is leaving NASA after 46 years. He tells the Post he wants to "spend full time on science, drawing attention to the implications for young people, and making clear what science says needs to be done."